Life is quite possible on Water Worlds

September 10, 2018

The conditions for life surviving on planets, entirely covered in water, are more fluid than previously thought, opening up the possibility that water worlds could be habitable.

Scientists thought that planets covered in a deep ocean would not support life assuming the absence of cycling of the minerals and gases keeping Earth’s climate stable. A study published in ‘The Astrophysical Journal’ used more than a thousand simulations to find that state ocean planets could stay in the “sweet spot” for habitability much longer than previously assumed. Edwin Kite, an Assistant Professor of Geophysical Sciences at the University of Chicago and the Lead Author of the study, said,

“This really pushes back against the idea you need an Earth clone — that is, a planet with some land and a shallow ocean.”

Advancements in telescopes have uncovered more planets, including water worlds orbiting stars in other solar systems. Scientists search for planets with both, water and some way to keep their climates stable over time as life needs an extended period to evolve. Similarly, light and heat on planets can change with the age of their host star. For instance, Earth cools itself over time by drawing down greenhouse gases into minerals and warms itself up by releasing them via volcanoes. Water worlds suppress volcanoes as their rock is submerged in deep water, making redundant the familiar Earth model.

The geoscientists of the Penn State suggest that plate tectonics (assumed a prerequisite for suitable conditions for life) are unnecessary. Consequently, a whole lot more habitable planets may exist in the universe than expected. Biosignatures of atmospheric Carbon Dioxide are looked for in the search for habitable planets or extra-terrestrial life. The existence of atmospheric Carbon Dioxide on Earth increases surface heat through the greenhouse effect. Natural processes also cycle Carbon to the subsurface and back to the atmosphere. Bradford Foley, an Assistant Professor of Geosciences, explained that in the following words:

“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment. Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”

Earth’s volcanoes are mainly found at the border of tectonic plates, which made scientists believe that they were necessary for life. Planets without tectonic plates, known as the ‘Stagnant Lid Planets’, have one giant, spherical plate of crust floating on the mantle, and are thought to be more abundant than planets with plate tectonics. Foley and Andrew Smye created a computer model of the lifecycle of a planet looking at how much heat its climate could retain based on its initial heat budget. The researchers found that at high enough heat and pressure, Carbon dioxide can escape from rocks and make its way to the surface, a process known as ‘Degassing’. Foley referred to that by saying,

“There’s a sweet spot range where a planet is releasing enough carbon dioxide to keep the planet from freezing over, but not so much that the weathering can’t pull carbon dioxide out of the atmosphere and keep the climate temperate.”

According to the researchers’ model, the presence and amount of heat-producing elements were far better indicators of a planet’s potential to sustain life. Kite teamed up with Eric Ford to set up a simulation using thousands of randomly generated planets and tracked the evolution of their climates over billions of years. They found that how much time a planet has is basically dependent on Carbon Dioxide and how it’s partitioned between the ocean, atmosphere, and rocks in its early years. It does seem there is a way to keep a planet habitable long-term without the geochemical cycling we see on Earth.

The lucky planets are located at the right distance from their stars, have the right quantity of carbon and don’t have too many minerals and elements from their crust dissolved in their oceans that would pull carbon out of the atmosphere. Kite mentioned that the simulations assumed stars that are like our own, but the results are optimistic for red dwarf stars, too. Planets in red dwarf systems are thought to be promising candidates for fostering life because these stars get brighter much slower than our sun (giving life a much longer time period to get started).

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